Mercury and arsenic and other heavy metals such as cadmium and lead are poisonous substances and are often present as soluble compounds in produced water from gas and oil recovery processes, and may be found in waste-water streams from refineries, flue gas washing and coal gasification. There is a need to find an environmentally sound and cost effective heavy metal removal technology.
U.S. Pat. No. 5,814,226 discloses a process for removing heavy metal ions from aqueous streams by contacting the streams with an inorganic ceramic support such as a silica gel on which has been supported a thiol or amine-functionality by means of a suitably functionalised organic silane or titanizing agent. No examples with mercury or arsenic are given.
Ind. Eng. Chem. Res. 2003, 42, 1955-1964 discloses mercury (II) ion adsorption from wastewaters using a thiol-functionalised adsorbent (“SOL-AD-IV”). The adsorbent is formed by co-hydrolysis of (3-mercaptopropyl) trimethoxy silane and tetraethoxysilane to form an adsorbent with specific physical characteristics.
J. Environ. Monit., 2003, 3, 366-370 discloses that mercury may also be absorbed from aqueous solutions using a thiol-functionalised silica material using a thiol-functionalised silane wherein the thiol-functionalised silane was formed on a silica gel surface by the reaction 3-mercaptopropyl silane and ethylene sulphide.
Thiol-functionalised adsorbents are however unstable, and the SOL-AD-IV adsorbent had to be stored under argon to prevent oxidation of the active thiols to inactive disulphides. Such storage would be prohibitively expensive and difficult on an industrial scale, even if an alternative inert gas was used. Furthermore the physical characteristics of the known products are not ideal for industrial scale use. We have further found that different physical characteristics in the supports can lead to adsorbents with improved heavy metal capture performance.